Low voltage emissive display (ED) technology based on organic LED (OLED), polymer LED (PLED), quantum dot, etc. offers great promise for improving display technology. EDs are beginning to appear on the market and break into the once-dominating LCD display technology for such things as portable electronic devices, computers, TVs, etc.
Currently, commercial EDs are relatively small and primarily based on passive technology, which is basically an array of overlapping conductors running in an X and Y direction. At each point of overlap between an X and Y conductor, sandwiched between the two conductors, is a layer of emissive material such that when current is driven through one X conductor and received at a Y conductor, the emissive material at the point of overlap of the two conductors illuminates due to the current flowing through it from the X to Y conductor. This is essentially a light emitting diode (LED). Efforts are continuing to manufacture larger displays for widespread use in such things as computer displays and TVs. These displays are preferably active matrix displays allowing each pixel to be individual addressed and driven rather than being addressed and driven by multiplexing (as in a passive conductor array). Advantages of active matrix displays are well known, with one of their biggest advantages being enhanced brightness.
Besides being cheaper, lighter, more flexible and consuming less power, ED technology offers other capabilities and advantages. For example, in addition to being optically emissive when electrically stimulated, many ED materials also exhibit photoelectric effect. That is, when light of sufficient energy strikes the emissive layer of the LED, there will be excitons (electron-hole pairs) created. Some of these excitons separate into their respective electrons and holes, and then each migrate to electrodes bounding the emissive layer, thus creating a potential difference between the electrodes. This potential difference or the magnitude of current flowing due to the charge migration can be sensed and gives and indication of the intensity of light illuminating the material. To improve the light detection sensitivity of the device, in many cases, it is preferable to apply a reverse bias to the LED (and in this mode, the leakage current or reverse bias resistance, which is dependent on the intensity of light striking the emissive layer, is sensed (i.e., measured)).
For example, U.S. Pat. No. 5,929,845 (incorporated herein by reference) purportedly describes a display/scanner combination based on a passive array of ED pixels where some pixels cast light while others are operable for scanning the emitted light reflected off an object to be scanned. US Patent Application Publication 2005/0231656 (incorporated herein by reference), has an image sensor having photosensitive TFT transistors in an array, but wherein the TFT devices themselves are not capable of generating or modulating light. TFT arrays are typically used to form active matrix displays. A similar device is shown in U.S. Pat. No. 6,831,710 (incorporated herein by reference). U.S. Pat. Nos. 6,947,102, 7,009,663, (both incorporated herein by reference), each show an LCD panel having light sensitivity, but wherein the light sensing element is not the light modulating portion of the pixel and is behind or off to the side of the light modulating element. U.S. Pat. No. 7,030,551 (incorporated herein by reference) purports to show a display pixel design which has an electroluminescence element as the light source and a photodiode as a light sensor, but wherein these are separate from one another and are purportedly made from different materials. See also U.S. Pat. Nos. 6,480,305, 6,404,137, 6,040,810, 5,446,564, and 4,972,252 (all of which are incorporated herein by reference) for further information on the state of the art and its progression.
Further information on ED materials and technologies can be found in Organic Light-Emitting Diodes: Principles, Characteristics and Processes, Kalinowski, CRC, November 2004; Organic Light-Emitting Devices, Shinar, Springer, October 2003; Organic Photovoltaics: Concepts and Realizations, Brabec, Springer, June 2003; Handbook of Polymers in Electronics, Malhotra, Rapra Technology Ltd, July 2002; Quantum Dots: Fundamentals, Applications, and Frontiers, Joyce et al, Springer, August 2005 (all of which are incorporated herein by reference).
One problem with the existing emissive display/optical scanner combination is that it is based on passive display technology, which limits its size and to some extent the brightness of the light emitting pixels (because of the need to multiplex the drivers to the pixels). The problem with existing light sensing LCD display technology is that it relies on the small (relative to the pixel size) TFT transistor to be the light sensing device and relies on ambient lighting to be the light source. The TFT transistor that receives the light for detection is susceptible to detecting internal reflections of the display's backlighting from the LCD material, and is generally not placed in an optimal position within the pixel window. Further, the optical TFT is usually beneath the LCD material and off to one side of the pixel, thus, reducing sensitivity.
The need exists for a display capable of generating and detecting light, wherein the generator and detector are preferably the same material so that the element's placement for display is also near optimal for light detection. The need also exists for such display/scanner combination that would preferably allow each pixel to operate as a light source or a light detector and, when acting as a light source, be capable of being decoupled from the display's drivers but still illuminate (i.e., be of the active matrix type). The need further exists for an active matrix-based ED display/scanner that is capable of generating light for illuminating an object to be scanned, and wherein the pixel itself (with its large window) is able to be used to detect reflected light (from an object or image being scanned) or ambient light. It would be beneficial if such a device, using the ED material itself as the light source and light detector, was capable of detecting and differentiating different wavelengths of light (such as red, green and blue for example) either by proper choice of ED materials or filtering of the light striking a pixel.